I wouldn't be surprised if many haven't noticed any of these observations yet -
In the "Exit" song by Little Chrissy and the Alphabeats, we see various Muppet characters in the audience who are gradually exiting the nightclub where the song is being performed. Apparently, one of the characters in the audience is Harvey Kneeslapper. He's sitting in the front row with two AM kids. I've always wondered who the character was with the light blonde hair, until I looked carefully and noticed his blue sweater (it's hard to notice with the distorted colors in the foreground due to the reddish tinted studio lights). Even though we don't actually see Harvey's face, and much less see him exiting the room during this segment, one other bit of evidence is the existence of the tall orange AM man with a red shirt as an audience patron. He's Harvey's victim in the "sticky 4" bit, which was very likely taped the same day as this "Exit" number.
In the "Some, None, and All" number with the Mahna-Mahna Muppet, there's a part in this segment where various Muppet monsters appear to demonstrate the song's key concept. One of the monsters is a really hideous-looking character with dark colored fur, orange eyes, a large blue pointed nose, and I believe a purple trim of facial fur and fanged teeth. Thankfully this character was never shown in a close up (that would be frightening beyond belief), but for those with a keen eye, you will notice that monster is ACTUALLY the original brown Grover puppet. Somehow, they still kept that puppet after season one, and decorated it with different features for this segment so it looks completely unrecognizable.
Also, has anyone ever noticed this? In Big Bird's home during much of the 1970s, there's a drawing of both Big Bird and Oscar the Grouch. It's seen hanging up on a dressing panel near Big Bird's nest. What makes this interesting is the fact that Oscar is colored orange in this picture. I can understand this picture was made during season one when Oscar was orange, but Big Bird's home wasn't seen during that season. It was implied and referred to, but we don't actually see his nest behind the fence of doors until the start of season two. What's interesting is the fact that by season two, Oscar was no longer orange, yet this picture is first seen on the set starting that season (by that time, Big Bird's home is now visible to the audience). I don't know if this is one of Caroll's own illustrations, or if it was drawn by someone else. Perhaps one of the viewers' artwork sent to the show. If not that picture of Big Bird and Oscar, then I have reason to believe this was artwork from one of the show's viewers', or else courtesy one of the children of the show's staff members -
in some of the very first few Bert and Ernie segments for season two, there's a crayon-like drawing of them on the wall. We can see that Ernie is yellow and Bert is red! Sure the colors are botched, but I guess if some youngster drew a picture of some of the shows' characters, then it was nice to include it as part of Bert and Ernie's home for it to be seen on TV.
There's another thing I've noticed in Big Bird's home for some time during the early 1970s. On the door fence scaffolding near his nest, there's a red slat with "123 1/2", which is supposedly Big Bird's home address. The additional fraction is an indication that Big Bird's home is clearly a shared division of the street's primary residence. If he has his own mailbox (it's blue with "B. BIRD" painted in yellow letters), then Big Bird should have his own home address.